Hydrocracking is a process which has achieved widespread use in petroleum refining for converting various petroleum fractions to lighter and more valuable products, especially distillates such as jet fuels, diesel oils and heating oils. Hydrocracking is generally carried out in conjunction with an initial hydrotreating step in which the heteroatom-containing impurities in the feed are hydrogenated without a significant degree of bulk conversion. During this initial step, the heteroatoms, principally nitrogen and sulfur, are converted to inorganic form (ammonia, hydrogen-sulfide) and these gases may be removed prior to the subsequent hydrocracking step although the two stages may be combined in cascade without interstage separation as, for example, in the Unicracking-JHC process and in the moderate pressure hydrocracking process described in U.S. Pat. No. 4,435,275.
In the second stage of the operation, the hydrotreated feedstock is contacted with a bifunctional catalyst which possesses both acidic and hydrogenation/dehydrogenation functionality. In this step, the characteristic hydrocracking reactions occur in the presence of the catalyst. Polycyclic aromatics in the feed are hydrogenated, and ring opening of aromatic and napthenic rings takes place together with dealkylation. Further hydrogenation may take place upon opening of the aromatic rings. Depending upon the severity of the reaction conditions, the polycyclic aromatics in the feed will be hydrocracked to paraffinic materials or, under less severe conditions, to monocylic aromatics as well as paraffins. Napthenic and aromatic rings may be present in the product, for example, as substituted napthenes and substituted polycyclic aromatics in the higher boiling products, depending upon the degree of operational severity.
The bifunctional catalyst typically comprises a metal component which provides the hydrogenation/dehydrogenation functionality and a porous, inorganic oxide support provides the acidic function. The metal component typically comprises a combination of metals from Groups IVA, VIA and VIIIA of the Periodic Table (IUPAC Table) although single metals may also be encountered. Combinations of metals from Groups VIA and VIIIA are especially preferred, such as nickel-molybdenum, cobalt-molybdenum, nickel-tungsten, cobalt-nickel-molybdenum and nickel-tungsten-titanium. Noble metals of Group VIIIA especially platinum or palladium may be encountered but are not typically used for treating high boiling feeds which tend to contain significant quantities of heteroatoms which function as poisons for these metals.
The porous support which provides the acidic functionality in the catalyst may comprise either an amorphous or a crystalline material or both. Amorphous materials have significant advantages for processing very high boiling feeds which contain significant quantities of bulky polycyclic materials (aromatics as well as polynapthenes) since the amorphous materials usually possesses pores extending over a wide range of sizes and the larger pores, frequently in the size range of 100 to 400 Angstroms (.ANG.) are large enough to provide entry of the bulky components of the feed into the interior structure of the material where the acid-catalyzed reactions may take place. Typical amorphous materials of this kind include alumina and silica-alumina and mixtures of the two, possibly modified with other inorganic oxides such as silica, magnesia or titania.
Crystalline materials, especially the large pore size zeolites such as zeolites X and Y, have been found to be useful for a number of hydrocracking applications since they have the advantage, as compared to the amorphous materials, of possessing a greater degree of activity, which enables the hydrocracking to be carried out at lower temperatures at which the accompanying hydrogenation reactions are thermodynamically favored. In addition, the crystalline catalysts tend to be more stable in operation than the amorphous materials such as alumina. The crystalline materials may, however, not be suitable for all applications since even the largest pore sizes in these materials, typically about 7.4 .ANG. in the X and Y zeolites, are too small to permit access by various bulky species in the feed. For this reason, hydrocracking of residuals fractions and high boiling feeds has generally required an amorphous catalyst of rather lower activity.
It would be desirable, if possible, to integrate the advantages of the amorphous and the crystalline material in hydrocracking catalysts and although the possibility of using active supports for crystalline materials has been proposed, the difference in activity and selectivity between the amorphous and crystalline materials has not favored the utilization of such catalysts.
The crystalline hydrocracking catalysts generally tend to produce significant quantities of gasoline boiling range materials (approximately 330.degree. F.-, 165.degree. C.-) materials as product. Since hydrocracked gasolines tend to be of relatively low octane and require further treatment as by reforming before the product can be blended into the refinery gasoline pool, hydrocracking is usually not an attractive route for the production of gasoline. On the other hand, it is favorable to the production of distillate fractions, especially jet fuels, heating oils and diesel fuels since the hydrocracking process reduces the heteroatom impurities characteristically present in these fractions to the low level desirable for these products. The selectivity of crystalline aluminosilicate catalysts for distillate production may be improved by the use of highly siliceous zeolites, for example, the zeolites possessing a silica:alumina ratio of 50:1 or more, as described in U.S. Pat. No. (Partridge et al), but even with this advance in the technology, it would still be desirable to integrate the characteristics of the amorphous materials with their large pore sizes capable of accommodating the bulky components of typical hydrocracking feeds, with the activity of the zeolite catalysts.